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Question 64 of 69

Q.Solve the equation 7x3−43x2=43x−77x^3-43x^2=43x-7

Puducherry TnboardTamil Nadu HSC (DGE) Board 2025Subjective· 3mImportance★★★★★
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Rewrites the equation with all terms on one side, recognises it as a palindromic (reciprocal) cubic which always has x=−1x=-1 as a root, divides it out, and solves the resulting quadratic.

  1. Rewrite 7x3−43x2=43x−77x^3-43x^2=43x-7 with all terms on one side: 7x3−43x2−43x+7=07x^3-43x^2-43x+7=0.
  2. The coefficients, in order, are 7, −43, −43, 77,\ -43,\ -43,\ 7 — reading them forwards or backwards gives the same sequence. This is a palindromic (reciprocal-type) equation.
  3. An odd-degree palindromic equation always has x=−1x=-1 as a root. Verify: 7(−1)3−43(−1)2−43(−1)+7=−7−43+43+7=07(-1)^3-43(-1)^2-43(-1)+7=-7-43+43+7=0 ✓.
  4. Divide 7x3−43x2−43x+77x^3-43x^2-43x+7 by (x+1)(x+1) using synthetic/long division: 7x3−43x2−43x+7=(x+1)(7x2−50x+7)7x^3-43x^2-43x+7=(x+1)(7x^2-50x+7). (Check: 7x2⋅x=7x37x^2\cdot x=7x^3; 7x2⋅1+(−50x)⋅x=7x2−50x2=−43x27x^2\cdot1+(-50x)\cdot x=7x^2-50x^2=-43x^2 ✓; −50x⋅1+7⋅x=−50x+7x=−43x-50x\cdot1+7\cdot x=-50x+7x=-43x ✓; 7⋅1=77\cdot1=7 ✓.) …

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