For the curve y = 4x3 – 2x5, find all the points at which the tangent passes through the origin.
The given curve y = 4x3 – 2x5
Then, the slope of the tangent at the point (x, y) is 12x2 – 10x4
The equation of the tangent at (x,y) is given by,
Y – y = (12x2 – 10x4)(X – x) ………….(1)
When the tangent passes through the origin (0,0), then X =Y=0
Then equation (1) becomes,
-y = (12x2 – 10x4)(– x)
⇒ y = (12x3 – 10x5)
Also, we have y = 4x3 – 2x5
⇒ (12x3 – 10x5) = 4x3 – 2x5
⇒ 8x5 – 8x3 = 0
⇒ x5 – 2x3 = 0
⇒ x3(x2 – 1) = 0
⇒ x = 0 , 1
When x = 0 then y = 0
When x = 1 then y = 2
And when x = -1 then y = -2
Therefore, the required points are (0, 0), (1,2) and (-1, -2).