For the curve y = 4x3 – 2x5, find all the points at which th

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 Multiple Choice QuestionsShort Answer Type

121.

Find points on the curve straight x squared over 4 plus straight y squared over 25 space equals space 1 at which the tangents are (i) parallel to the x-axis (ii) parallel to the y-axis.

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 Multiple Choice QuestionsLong Answer Type

122.

Find points on the curve straight x squared over 9 plus straight y squared over 16 space equals space 1 at which the tangents are (i) parallel to the x-axis (ii) parallel to the y-axis.

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123.

For the curve y = 4x3 – 2x5, find all the points at which the tangent passes through the origin.


The equation of curve is
                               y = 4x3 – 2x5                                               ...(1)
Let the tangent to the curve (1) at (h, k) pass through origin (0, 0)
                  therefore space space space straight k space equals space 4 straight h cubed minus 2 straight h to the power of 5                                                 ...(2)
                                                                         open square brackets because space space space space left parenthesis straight h comma space straight k right parenthesis space lies space on space left parenthesis 1 right parenthesis close square brackets
Differentiating (1) w.r.t.x, dy over dx space equals space 12 straight x squared minus 10 straight x to the power of 4
At space left parenthesis straight h comma space straight k right parenthesis comma space space dy over dx space equals space 12 straight h squared minus 10 straight h to the power of 4 comma space space which space is space slope space of space tangent.
The equation of tangent of (h, k) is 
                        straight y minus straight k space equals space left parenthesis 12 straight h squared minus 10 straight h to the power of 4 right parenthesis thin space left parenthesis straight x minus straight h right parenthesis
or        straight y equals left parenthesis 4 straight h cubed minus 2 straight h to the power of 5 right parenthesis space equals space left parenthesis 12 straight h squared minus 10 straight h to the power of 4 right parenthesis thin space left parenthesis straight x minus straight h right parenthesis space space space space space space space open square brackets because space of space left parenthesis 2 right parenthesis close square brackets
Now this tangent passes through (0, 0).
therefore space space space space space space 0 minus left parenthesis 4 straight h cubed minus 2 straight h to the power of 5 right parenthesis space equals space left parenthesis 12 straight h squared minus 10 straight h to the power of 4 right parenthesis thin space left parenthesis 0 minus straight h right parenthesis space space space space space space space space
or     negative 4 straight h cubed plus 2 straight h to the power of 5 space equals space minus 12 straight h cubed plus 10 straight h to the power of 5 space space space space or space space space space 8 straight h to the power of 5 minus 8 straight h cubed space equals space 0
or       straight h to the power of 5 minus straight h cubed space equals space 0 space space space space space or space space space straight h cubed left parenthesis straight h squared minus 1 right parenthesis space equals space 0.
rightwards double arrow              straight h space equals space 0 comma space space space 1 comma space space space minus 1
therefore space space space space space straight k space equals space 0 comma space space 4 minus 2 comma space space minus 4 plus 2 space space space space space space space space space space space space space space space space space space space space space space space space space space space space open square brackets because space of space left parenthesis 2 right parenthesis close square brackets
space space space space space space space space space space space space space equals space 0 comma space space 2 comma space space minus 2
therefore space required space points space are space left parenthesis 0 comma space 0 right parenthesis comma space left parenthesis 1 comma space 2 right parenthesis comma space left parenthesis negative 1 comma space minus 2 right parenthesis

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 Multiple Choice QuestionsMultiple Choice Questions

124. The points on the curve 9y2 = x3, where the normal to the curve make equal intercepts with the axes are
  • open parentheses 4 comma space plus-or-minus 8 over 3 close parentheses
  • open parentheses 4 comma space space minus 8 over 3 close parentheses
  • open parentheses 4 comma space plus-or-minus 3 over 8 close parentheses
  • open parentheses 4 comma space plus-or-minus 3 over 8 close parentheses
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 Multiple Choice QuestionsLong Answer Type

125. The curve y = ax3 + bx2 + cx + 5 touches the x -axis at P (– 2, 0) and cuts the y-axis at a point Q where its gradient is 3. Find a. b, c.
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126.

Show that the curves 2x = y2 and 2xy = k cut at right angles if k2 = 8

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127. Prove that the curves x = y2 and xy = k cut at right angles if 8k2 = 1.
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128.

Show that the curve xy = a2 and x2 + y2 = 2a2 touch each other.

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129.

If the curve αx+ βy2 = 1 and α' x+ β'y2 = 1 intersect orthogonally, prove that (α – α') β β') = (β – β') α α'. 

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 Multiple Choice QuestionsMultiple Choice Questions

130. The slope of the tangent to the curve x = t2 + 3t – 8 , y = 2t2 – 2t – 5 at the point (2, – 1) is
  • 22 over 7
  • 6 over 7
  • 7 over 6
  • 7 over 6
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