a6+a9 = 90 = a1
we know that:
a6 = a2+4r
==> a2+4r + a9 = 90-a1
==> a1+a2 + a9 + 4r = 90
==> a1+ a2 + a9 = 90-4r .......(1)
Now we need the values of:
a1+a2 + a10 = ??
a10 = a9 + r
==> a1 + a2 + a10 = a1 + a2 + a9 + r
From (1) :
= 90-4r + r
= 90-3r
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