Differentiate the functions given in question 1 to 10 with respect to x.
Question 1. cos x.cos2x.cos3x
Solution:
Let us considered y = cos x.cos2x.cos3x
Now taking log on both sides, we get
log y = log(cos x.cos2x.cos3x)
log y = log(cos x) + log(cos 2x) + log (cos 3x)
Now, on differentiating w.r.t x, we get
= -y(tan x + 2tan 2x + 3 tan 3x)
= -(cos x. cos 2x. cos 3x)(tan x + 2tan 2x + 3tan 3x)
Question 2.
Solution:
Let us considered y =
Now taking log on both sides, we get
log y =
log y =
(log(x – 1)(x – 2)(x – 3)(x – 4)(x – 5)) log y =
(log(x – 1) + log(x – 2) – log(x – 3) – log(x – 4) – log(x – 5)) Now, on differentiating w.r.t x, we get
Question 3. (log x)cos x
Solution:
Let us considered y = (log x)cos x
Now taking log on both sides, we get
log y = log((log x)cos x)
log y = cos x(log(log x))
Now, on differentiating w.r.t x, we get
Question 4. xx – 2sin x
Solution:
Given: y = xx – 2sin x
Let us considered y = u – v
Where, u = xx and v = 2sin x
So, dy/dx = du/dx – dv/dx ………(1)
So first we take u = xx
On taking log on both sides, we get
log u = log xx
log u = x log x
Now, on differentiating w.r.t x, we get
du/dx = u(1 + log x)
du/dx = xx(1 + log x) ………(2)
Now we take v = 2sin x
On taking log on both sides, we get
log v = log (2sinx)
log v = sin x log2
Now, on differentiating w.r.t x, we get
dv/dx = v(log2cos x)
dv/dx = 2sin xcos xlog2 ………(3)
Now put all the values from eq(2) and (3) into eq(1)
dy/dx = xx(1 + log x) – 2sin xcos xlog2
Question 5. (x + 3)2.(x + 4)3.(x + 5)4
Solution:
Let us considered y = (x + 3)2.(x + 4)3.(x + 5)4
Now taking log on both sides, we get
log y = log[(x + 3)3.(x + 4)3.(x + 5)4]
log y = 2 log(x + 3) + 3 log(x + 4) + 4 log(x + 5)
Now, on differentiating w.r.t x, we get
Question 6.
Solution:
Given: y =
Let us considered y = u + v
Where
and so, dy/dx = du/dx + dv/dx ………(1)
Now first we take
On taking log on both sides, we get
log u =
log u =
Now, on differentiating w.r.t x, we get
………(2) Now we take
On taking log on both sides, we get
log v =
log v =
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 7. (log x)x + x log x
Solution:
Given: y = (log x)x + x log x
Let us considered y = u + v
Where u = (log x)x and v = xlog x
so, dy/dx = du/dx + dv/dx ………(1)
Now first we take u = (log x)x
On taking log on both sides, we get
log u = log(log x)x
log u = x log(log x)
Now, on differentiating w.r.t x, we get
………(2) Now we take v = xlog x
On taking log on both sides, we get
log v = log(xlog x)
log v = logx log(x)
log v = logx2
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 8. (sin x)x + sin–1√x
Solution:
Given: y = (sin x)x + sin–1√x
Let us considered y = u + v
Where u = (sin x)x and v = sin–1√x
so, dy/dx = du/dx + dv/dx ………(1)
Now first we take u = (sin x)x
On taking log on both sides, we get
log u = log(sin x)x
log u = xlog(sin x)
Now, on differentiating w.r.t x, we get
………(2) Now we take v = sin–1√x
On taking log on both sides, we get
log v = log sin–1√x
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 9. x sin x + (sin x)cos x
Solution:
Given: y = x sin x + (sin x)cos x
Let us considered y = u + v
Where u = x sin x and v = (sin x)cos x
so, dy/dx = du/dx + dv/dx ………(1)
Now first we take u = x sin x
On taking log on both sides, we get
log u = log xsin x
log u = sin x(log x)
Now, on differentiating w.r.t x, we get
………(2) Now we take v =(sin x)cos x
On taking log on both sides, we get
log v = log(sin x)cos x
log v = cosx log(sinx)
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 10.
Solution:
Given: y =
Let us considered y = u + v
Where u = xxcosx and v =
so, dy/dx = du/dx + dv/dx ………(1)
Now first we take u = xxcosx
On taking log on both sides, we get
log u = log (x xcosx)
log u = x.cosx.logx
Now, on differentiating w.r.t x, we get
………(2) Now we take v =
On taking log on both sides, we get
log v = log
log v = log(x2 + 1) – log(x2 – 1)
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 11. Differentiate the function with respect to x.
(x cos x)x + (x sin x)1/x
Solution:
Given: (x cos x)x + (x sin x)1/x
Let us considered y = u + v
Where, u = (x cos x)x and v = (x sin x)1/x
So, dy/dx = du/dx + dv/dx ………(1)
So first we take u = (x cos x)x
On taking log on both sides, we get
log u = log(x cos x)x
log u = xlog(x cos x)
Now, on differentiating w.r.t x, we get
………(2) Now we take u =(x sin x)1/x
On taking log on both sides, we get
log v = log (x sin x)1/x
log v = 1/x log (x sin x)
log v = 1/x(log x + log sin x)
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Find dy/dx of the function given in questions 12 to 15
Question 12. xy + yx = 1
Solution:
Given: xy + yx = 1
Let us considered
u = xy and v = yx
So,
………(1) So first we take u = xy
On taking log on both sides, we get
log u = log(xy)
log u = y log x
Now, on differentiating w.r.t x, we get
………(2) Now we take v = yx
On taking log on both sides, we get
log v = log(y)x
log v = x log y
Now, on differentiating w.r.t x, we get
………(3) Now put all the values from eq(2) and (3) into eq(1)
Question 13. yx = xy
Solution:
Given: yx = xy
On taking log on both sides, we get
log(yx) = log(xy)
xlog y = y log x
Now, on differentiating w.r.t x, we get
Question 14. (cos x)y = (cos y)x
Solution:
Given: (cos x)y = (cos y)x
On taking log on both sides, we get
y log(cos x) = x log (cos y)
Now, on differentiating w.r.t x, we get
Question 15. xy = e(x – y)
Solution:
Given: xy = e(x – y)
On taking log on both sides, we get
log(xy) = log ex – y
log x + log y = x – y
Now, on differentiating w.r.t x, we get
Question 16. Find the derivative of the function given by f(x) = (x + 1)(x + x2)(1 + x4)(1 + x8) and hence find f'(1).
Solution:
Given: f(x) = (x + 1)(x + x2)(1 + x4)(1 + x8)
Find: f'(1)
On taking log on both sides, we get
log(f(x)) = log(1 + x) + log(1 + x2) + log(1 + x4) + log(1 + x8)
Now, on differentiating w.r.t x, we get
∴ f'(1) = 2.2.2.2.
f'(1) = 120
Question 17. Differentiate (x5 – 5x + 8)(x3 + 7x + 9) in three ways mentioned below
(i) By using product rule
(ii) By expanding the product to obtain a single polynomial
(iii) By logarithmic differentiation.
Do they all give the same answer?
Solution:
(i) By using product rule
dy/dx = (3x4 – 15x3 + 24x2 + 7x2 – 35x + 56) + (2x4 + 14x2 + 18x – 5x3 – 35x – 45)
dy/dx = 5x4 – 20x3 + 45x2 – 52x + 11
(ii) By expansion
y = (x2 – 5x + 8)(x3 + 7x + 9)
y = x5 + 7x3 + 9x2 – 5x4 – 35x2 – 45x + 8x3 + 56x + 72
y = x5 – 5x4 + 15x3 – 26x2 + 11x + 72
dy/dx = 5x4 – 20x3 + 45x2 – 52x + 11
(iii) By logarithmic expansion
Taking log on both sides
log y = log(x2 – 5x + 8) + log(x3 + 7x + 9)
Now on differentiating w.r.t. x, we get
dy/dx = 2x4 + 14x2 + 18x – 5x3 – 35x – 45 + 3x4 – 15x3 + 24x2 + 7x2 – 35x + 56
dy/dx = 5x4 – 20x3 + 45x2 – 52x + 11
Answer is always same what-so-ever method we use.
Question 18. If u, v and w are function of x, then show that
Solution:
Let y = u.v.w.
Method 1: Using product Rule
Method 2: Using logarithmic differentiation
Taking log on both sides
log y = log u + log v + log w
Now, Differentiating w.r.t. x