Please solve question number 10 and 12(the ones which are encircled).

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Please solve question number 10 and 12(the ones which are
encircled)

Neha Sethi · Accepted Answer

Dear student
Q 10
Last term,l=a+(n-1)dSn=n22a+9n-1)dNow, 8ds=d+2l2⇒8×d×n22a+(n-1)d=d+2a+(n-1)d2⇒4dn2a+nd-d=d+2a+2dn-2d2⇒4dn2a+nd-d=2a+2dn-d2⇒8adn+4d2n2-4d2n=4a2+4d2n2+d2+8adn-4d2n-4ad⇒4a2+d2-4ad=0⇒2a-d2=0⇒2a-d=0⇒d=2a
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